Effect of ignition chemistry on turbulent premixed flames of n-heptane and toluene

Turbulent premixed flames of n-heptane/air and toluene/air mixtures have been experimentally investigated in a reactor-assisted turbulent S_Lot (RATS) burner at two burner temperatures, 450 K and 650 K, by measuring turbulent burning velocities (S_T), flashback, and flame structures with planar laser-induced fluorescence (PLIF) imaging at various equivalence ratios (φ). Turbulent burning velocities have been found to be affected by the low-temperature-ignition for n-heptane/air mixture due to two-stage ignition process, whereas no LTI affected turbulent burning velocity is found in case of toluene/air mixture due to the absence of two-stage ignition process. The measured turbulent burning velocities of n-heptane and toluene at 450 K exhibits identical trend of u’/S_L dependency, once they are normalized by the laminar burning velocities (S_L), indicating appropriate representation of chemical effect by S_L. In order to investigate the effect of Lewis number (Le), turbulent burning velocities and flame structure are measured by changing the equivalence ratio from 0.7 to 1.5 at 450 K. The results show that S_T is insensitive to the change of Le at fuel lean, however strong dependency of S_T on Lewis number is found at fuel rich conditions due to the increase of flame front wrinkling. Flame flashback conditions are measured in a function of mean jet velocity and equivalence ratio for n-heptane/air mixtures from lean (φ = 0.7) to rich (2.1) cases. The flashback measurements reveal completely different behaviors, depending on the burner temperature. At 450 K (chemically-frozen regime) the flashback is found to be controlled by the turbulent burning velocity, whereas at 650 K (Ignition-driven regime) the flashback is correlated well with the calculated hot-ignition delay times of n-heptane/air mixture. In ignition-driven regime, the measured S_T/S_L increases substantially as increasing the ignition Damkőhler number.